Will I be able to
Do you mean, will something physically stop you from being able to? No
what is the worst that can happen to my laptop with the original OEM key
The worst that can happen is that the key is invalidated by Microsoft, and that any computer that uses it displays a notification that "This copy of Microsoft Windows is not Genuine"... and needs to be reinstalled every 30 days, and/or can't be updated, etc. So, you can invalidate the new install and your old install. Essentially, you can make the OEM product key invalid for any and every installation.
And my brother did this for the other computer
With the OEM key from the bottom? Because here's the deal. When you get your laptop and start using it, there is one product key on the sticker on the bottom of your laptop... and your laptop is currently installed with a DIFFERENT product key called a Volume License product key. This is a single product key that the laptop manufacturer used on thousands of laptops when they made the hard drive image for mass production. However, the manufacturer is required by law to provide you with a UNIQUE and individual product key, since you are also purchasing a copy of Windows with your laptop. So, provided you haven't had to reinstall Windows yet, you actually have never used the OEM key on the sticker.
(EDIT This is easy enough to confirm yourself. Just download any keyfinder software that will pull the MS product key for your installation from your installation, and compare it to the OEM product key on the sticker. If you have never reinstalled Windows on that machine, you will find them to be different. Go Ahead. Try it. Use RockXP or ProduKey or Magical Jellybean.)
So... Can you use the OEM product key on a second installation of Windows, provided you have NEVER used it on the computer it came with originally? Yes. Is this recommended? No. You know why? What are you going to do when you need that OEM product key for the computer it came with? Oh... don't give me that "It's not gonna happen." Don't blow off the real probability that your hard drive is going to fail in a year or two... and you will be kicking yourself because you didn't make the recovery discs that you were supposed to (or they don't work right like so many people complain here about).
... and of course, as has been pointed out very succinctly... it is illegal too.